1. Field of the Invention
The invention relates to the field of microelectromechanical systems (MEMS), and in particular to micromachined anemometers.
2. Description of the Prior Art
A hot-wire anemometer is a thermal transducer which is capable of sensing point flow velocity by means of temperature variations using a heated resistive wire which is a nonzero temperature coefficient of resistance. When the electrically heated wire is placed in flow of fluid, heat is taken away by flow-induced forced convection. Depending upon the operational mode used, e.g. constant current or constant temperature, either the resistance or the voltage output drop across the wire is then a function of the flow velocity.
Conventional hot-wire anemometers have been used for flow velocity measurements for more than 80 years. The conventional structure is a metal wire, welded or soldered between two metal needles, which are molded to a probe body. The wire is usually made of platinum or tungsten and is typically 5 microns in diameter and 1 millimeter in length. The wire typically has a resistance of 10 to 30 ohms at room temperature and requires 10 to 40 milliamps of current to operate. Conventional anemometers are usually hand-assembled, thus making it difficult and expensive to incorporate them into large arrays for simultaneous velocity distribution measurements. Also, since the wire diameter is difficult to control with good repeatability, the anemometer probes are essentially noninterchangeable without recalibrating the anemometer system.
Since the spatial resolution of the anemometer for flow velocity distribution measurements is determined by the anemometer's dimensions, it is advantageous if the wire size could be reduced. This would reduce power consumption and thermal interference to the flow and increase frequency response. In fact, many prior art anemometer designs have been demonstrated using either surface or bulk micromachining technologies. See, Y. C. Tai et al., "Polysilicon Bridge for Anemometer Application," Digest Tech Papers, Transducers '85, Philadelphia, Pa., Jun. 4-7, 1985 at 354-57; H. Rahnamai et al., "Pyroelectric Anemometers: Preparation of Velocity Measurements," Sensors and Actuators, Volume 2, at 3-16 (1981); M. Stenberg et al., "A Silicon Sensor for Measurement of Liquid Flow and Thickness of Fouling Biofilms," Sensors and Actuators, Volume 13 at 203-21 (1988); B. W. Van Oudheusden et al., "Integrated Silicon Flow Direction Sensor, " Sensors and Actuators, Volume 16, at 109-19 (1989); and R. Kersjes et al., "An Integrated Sensor for Invasive Blood-Velocity Measurement," Sensors and Actuators, Volume 37-38 at 674-78 (1993).
Interestingly, however, these anemometers are either bulk micromachined chip-size devices or wires on top of chips. No one has been able to simulate the features of a conventional hot-wire, that is a wire free-standing in space without anything nearby so that good thermal isolation is achieved. As a result, the prior art anemometers are not direct replacements for conventional hot-wire anemometers.
Therefore, what is needed is a new type of micromachined anemometer that is capable of performance simulation of conventional hot-wire anemometers, but with a greatly reduced wire size, improved spatial resolution, improved device sensitivity, and improved frequency response.
In order to modify a surface to reduce drag or to control vortices within the boundary layer above the surface, sensors must be able to provide flow field information having a spatial resolution of the order of 100 microns and a frequency response of greater than a kHz. Shear stress sensors previously used exploit either a direct or indirect method for shear stress detection. In the direct method, a tangential force on a surface floating balance gives a direct measurement to the shear stress. In the indirect method, shear stress is extracted from other physical measurands that are indirectly related to shear stress such as Preston tubes, Stanton tubes and hot-wire/film surface mounted sensors. These conventional shear stress sensors, however, can only be hand-made, one at a time, and therefore ill-adapted for use in a system where thousands of uniform sensors are required.
In response, the prior art has developed micromachined surface floating balance for direct shear stress measurements. Sensitivity is 52 microvolts/Pa for gas sensing and 13.7 microvolts per volt-kPa for liquid sensing have been reported using a piezo resistive readout. Flow sensors based on heat transfer principles have also been demonstrated in micromachined sensors using free-standing beams, free-standing diaphragms and low thermal conductivity layers such as polyimid. Unfortunately, none of these devices are adaptable for use in large array vortices control within a boundary layer.
Therefore, what is needed some type of shear stress sensor adaptable for use in large arrays for vortice control within a boundary layer.